In general, exercises with more repetitions are performed for the improvement of muscular endurance, while those with a higher weight and fewer repetitions are practiced to increase muscle size and strength. Here are some guidelines to start. You may want to work with a personal trainer or other professional to develop a program that allows you to achieve your goals.
High repetitions, medium repetitions, low repetitions ... all approaches are promoted as an ideal way to develop muscle, although the conclusions are not always clarifying. In Sports and Life we will see the cases separately and we will analyze which is the best option.
High repetitions (15 or more)
However, it is an excellent means to increase muscular endurance, and therefore, can be very useful in many sports that have nothing to do with hypertrophy (muscle mass gain). However, if the essential thing is size, high repetitions are not a good idea: neither to grow, nor to define.
Low repetitions (5 or less)
It is a very safe method to build strength, and of course, there is more to see the size of some lifters like 'The Mountain', Game of Thrones. However, low repetition training has one major drawback: the stimulation of muscle fiber and, therefore, growth, which correlates closely with the amount of time a muscle is under tension. Short and intense sets of 15 seconds or less will develop strength, but are not as effective when hypertrophying.
Moderate repetitions (between 8 and 12)
When lactic acid, or lactate, accumulates in large amounts, it induces an increase in the levels of anabolic hormones in the body, including growth hormone and the mother of muscle development, testosterone.
The range of moderate repetitions, when combined with a challenging weight, also causes a very desired condition: the muscular pumping. It is something like that feeling of tightness and fullness under the skin, caused by the accumulation of blood in the muscle. Studies have shown that the physiological conditions that lead to the muscular pumping activate the synthesis of proteins, and therefore, the muscular growth.
In the final analysis, substantial evidence holds that training in a range of moderate repetition (8-12 reps) is the best way to develop muscle mass. It increases the hormonal response, stimulates protein synthesis and provides the necessary time under tension to cause muscle damage.
Of course, to ensure that your body does not adapt to a particular regime and stagnate, you need variety. Perform low repetition training cycles to gain strength and use high repetition training to improve your endurance. Everything adds up, after all, and depends on your goals, but to gain muscle, it is best to keep in a moderate range of repetitions.